The invention relates to a latency time circuit for an S-DRAM according to the precharacterizing clause of Patent claim 1.
D-RAM modules are standard memory modules for main memories. D-RAM memories are composed of large-scale integrated transistors and capacitors. In order to obtain the information, the memory contents must be continuously refreshed in this case. A synchronous D-RAM (S-DRAM) allows memory access without any additional waiting cycles. In this case, the data is transferred between the S-DRAM and an external data bus in synchronism with the external clock signal.
FIG. 1 shows an S-DRAM memory module according to the prior art. The S-DRAM memory module is connected to an external control bus, to an external address bus and to an external data bus. The control commands which are applied to the external control bus are read in via command PADS by means of an integrated command receiver, and the received signals are applied, after signal amplification, to a command decoder. The command decoder decodes the applied control commands which, by way of example, have a length of 4 bits, to form internal control commands, for example write (WR) and read (RD). The S-DRAM contains a state machine or sequence controller which controls the internal sequences as a function of the decoded internal control commands. The sequence controller is clocked by a clock signal. For this purpose, an external clock signal CLKext is applied to the S-DRAM, and the signal is amplified by means of an integrated clock signal receiver. The amplified clock signal is distributed in the form of a tree in the integrated S-DRAM by means of a clock tree, and is passed via an internal clock line to a sequence controller. The external clock signal is furthermore applied to a delay locked loop DLL. The delay locked loop DLL causes a negative phase shift in the applied external clock signal CLK. The internal DLL clock signal precedes the external clock signal in order that the data is applied to the data pads in synchronism with the external clock signal. The DLL clock signal DLLCLK is used for clocking the output signal driver OCD (Off Chip Driver), which is integrated in the S-DRAM, for one data path. The delay locked loop DLL is followed by a delay element which forms an internal clock signal (VE-CLK) which is modeled such that it is identical to the external clock signal, that is to say VE-CLK is completely in synchronism with CLKext. The delay element for this purpose compensates for the negative phase shift in the delay locked loop DLL.
The internal sequence controller produces control signals for the internal processing sequence of the S-DRAM as a function of the decoded commands. The sequence controller generates an RAS signal (Row Address Strobe) for driving a row address latch, and a CAS signal (Column Address Select) for driving a column address latch. The row address latch and the column address latch are connected via an internal address bus to an address signal receiver for the S-DRAM. Via the external address bus, the S-DRAM receives an external address at the address PADS, with the applied address signals being amplified by an address receiver. In order to save connections, the address is entered in DRAM memories in two steps. In a first step, the lower address bits are loaded together with the RAS signal into the row address latch. In a second step, the most significant address bits are loaded, together with the CAS signal, into the column address latch. The address bits are applied to a row or column decoder, respectively, for access to a memory row within the memory cell array, which is in the form of a matrix. The row address latch and the column address latch as well as the row and column decoders together form an address signal decoder. In order to refresh the memory cells, the memory cell array receives a refresh control signal from the sequence controller. A refresh counter, which receives an enable signal from the sequence controller, produces all the existing row addresses successively, and these are then applied to the address bus. For this purpose, the sequence controller produces an RAS control signal. A word line is activated in order to refresh all the memory cells which are connected to it.
The memory cell array is also connected to read/write amplifiers. The number of read/write amplifiers depends on the memory architecture, on the word length and on the prefetch. In the case of a prefetch 4 with a word length of 32, for example, 128 read/write amplifiers are in operation at the same time. If, by way of example, four independent memory banks are provided, a total of 512 read/write amplifiers are integrated on the memory chip.
One data bit is in each case written to an addressed memory cell, or is read from it, via the read/write amplifiers. The read/write amplifiers are connected via an internal data bus to an internal bus path in the S-DRAM. The data in the external data bus is written synchronously via the data path to the S-DRAM, and is emitted synchronously from the S-DRAM. The data path is connected to the data PADS of the S-DRAM.
In order to read data, the data path has a data receiver for receiving the externally applied data. An internal driver circuit for the data to be read (WR driver) amplifies the signals in the received data and emits the data that has been read via the internal bus to the read/write amplifiers. The driver circuit WR driver is driven by a write/latency time generator, which is clocked by the internal clock signal VE-CLK. The write/latency time generator is itself connected to a decoder.
For synchronous data emission, the data path contains a data FIFO register, which is followed by an output data driver circuit (OCD driver). The FIFO register is driven by the read/write amplifier by means of an input pointer and by a read/latency generator by means of an output pointer or a delayed data enable signal. The read/latency generator is likewise connected to a decoder.
The two decoders for the read/latency time generator and for the write/latency time generator are connected via internal control lines to a mode register, in which the data for controlling the operating modes is stored within the S-DRAM. The mode register can be initialized by means of a mode register set command via the internal address bus. The mode register is initialized after switch on. Before any external control commands are applied to the S-DRAM, the mode register is initialized. The mode register contains control data for the CAS latency time, for test modes and for a DLL reset.
The sequence controller generates an internal write command PAW as a function of the external control commands, in order to activate the write/latency time generator, and generates an internal read command PAR for activation of the read/latency time generator.
FIG. 2 shows a timing diagram to explain the method of operation of a conventional S-DRAM. An external clock signal CLK-external is applied to the S-DRAM. Depending on the decoded read command RD, the state machine or sequence controller generates an internal read command signal PARint. The read command is applied relative to a rising signal flank by the external clock signal CLKext. The clock signal is received and distributed. The internal clock signal CLKint is used to transfer the command, and then to decode it. The sequence controller produces, for example, an internal clock signal PARint.
The internal clock signal PARint is generated with a certain signal delay, namely a decoding time xcex94tDEC. This decoding time comprises a signal delay resulting from the clock signal receiver, on the basis of the clock signal line tree (clock tree) and on the basis of signal delays within the sequence controller.
tDEC=tCLKRECEIVERT+tCKLTREE+tLatch+tCMDDecode+tPARGeneration
The generated internal read signal PARint is applied with a short signal delay to the read/write amplifiers, which emit the data to be read out to the internal data bus. The data is passed with the further time delay xcex94tFIFO from the internal data bus via the FIFO register within the data path to the input of the OCD driver. The OCD driver, or data output driver, emits the data with a further signal delay xcex94tOCD to the data PADS of the S-DRAM. There is a delay time At between the flank of the external clock signal at which the decoded internal read command RD is applied and the data output via the data PADS.
FIG. 3 shows a latency time generator according to the prior art, which is contained in the data path of the conventional S-DRAM. The conventional latency time generator illustrated in FIG. 3 receives the internal read signal PARint from the sequence controller. The internal read signal is synchronized to the DLL clock by means of a synchronization circuit, which comprises a first synchronization latch A and a second synchronization latch B. The clock signal VE-CLK is completely synchronized to the external clock signal CLK-external. In a first step, PAR is synchronized to the VECLK (latch A) and, in a second step, it is synchronized to the DLL-CLK. The internal read signal PARxe2x80x3int that has been synchronized in this way is applied to a chain of series-connected time switching elements, each of which causes a delay of one clock cycle. The time switching elements are clocked by the clock signal DLL-CLK. Each of the time switching elements produces a signal delay which is identical to the cycle time of the xcex94tcycle of the external clock signal.
In the read/latency time generator as illustrated in FIG. 3 and according to the prior art, the CAS latency time is stored in the mode register, in which case, by way of example, it is possible to store a CAS latency time of 6, a CAS latency time of 5 and a CAS latency time of 4 in the mode register. Memories also exist with other latency times, for example 2, 3 or 7. The read latency time or CAS latency time indicates a number of clock cycles between the application of the external read command and the appearance of the emitted data at the OCD driver. The expression read latency means the number of clock cycles which pass between application of a read command to a synchronous memory and the appearance of the requested data at the output of the memory. A short read latency has the advantage that a connected controller requires fewer waiting cycles. Depending on the lengths of the internal signal delay times, decoding times and amplifier delay times on the memory chip, a short read latency can be achieved, or longer read latency times must be accepted. One important influencing factor in this case is the quality of the production process. These process fluctuations, however, not only govern the read latencies of the memory chip but are also a main influencing factor for the capability of the memory chip to achieve high clock rates.
The expression write latency means the number of clock cycles which pass between the application of a write command and the application of the data to the inputs of the memory chip. The read latency time for conventional S-DRAMs can be programmed into a mode register. The write latency in the case of the DDR2 Standard is coupled to the read latency and is one clock cycle less than the read latency.
Write Latency=Read Latencyxe2x88x921.
In order to achieve a read latency time or CAS latency time of CAS=6, the synchronized internal read signal PARxe2x80x3int is applied to the chain of timing elements and is delayed with a delay which corresponds to four times the clock cycle time xcex94tcycle. In order to achieve a CAS latency time of 5, the synchronized internal read signal passes through only three timing elements, and, in order to achieve a CAS latency time of 4, the synchronized internal read signal passes through only two timing elements. In a corresponding manner, three inputs are provided on an internal multiplexer for the latency time generator, and are connected to outputs of timing elements within the chain. The decoder decodes the desired CAS latency time, which is stored digitally in the mode register, and drives the multiplexer via a control line. If, for example, a CAS latency time of 4 is stored in the mode register, the decoder connects the third input of the multiplexer to the output control line. The multiplexer is connected on the output side to the FIFO register, and emits a delayed enable signal to the FIFO register.
FIG. 4 shows a timing diagram to explain the function of the latency time generator or latency time counter according to the prior art. The example illustrated in FIG. 4 shows the procedure for a stored CAS latency time of 4. The decoder identifies the CAS latency time of 4, and connects the third input of the multiplexer, so that a time delay is produced by two clocked timing elements. Since the timing elements are clocked by the internal clock signal DLL-CLK, the data is enabled with a time delay after the fourth rising flank of the DLL-CLK clock signal. The internal read command signal which is applied to the latency time generator is in fact passed to the latency time generator only after a time delay of xcex94tDEC. 
As the clock frequency of the external clock signal rises, the cycle time tcycle of the clock signal decreases. If the clock rate is 500 MHz, the cycle time tcycle is now only 2 ns, and is in the same order of magnitude as the signal delay times on the chip. Since the time delay xcex94tDEC is constant, a situation occurs when the clock signal frequency is very high in which the signal delay xcex94tDEC is greater than the cycle time tcycle. If the signal delay xcex94tDEC is greater than the cycle time, the signal PAR is synchronized to PARxe2x80x2 with the second VE-CLK signal flank, instead of with the first VE signal flank.
If there is a time offset between DLL-CLK and VE-CLK which is greater than one clock cycle time tcycle, the synchronization of PARxe2x80x3int to PARxe2x80x3int takes place with the second DLL-CLK signal flank instead of with the first DLL-CLK flank or, if the first failure mechanism has already occurred, with the DLL-CLK signal flank three of the DLL-CLK flank 2.
The read latency counter according to the prior art thus switches a very high-frequency applied clock signal one clock signal too late, and the S-DRAM incorrectly emits the data too late. This in turn leads to considerable malfunctions of the overall system, in particular of the microprocessor which is connected to the S-DRAM.
FIG. 5 shows the read access to an S-DRAM memory.
FIG. 5a shows the situation where the external clock signal is at a relatively low frequency, and the data is emitted correctly by the S-DRAM.
FIG. 5b shows the situation for a very high-frequency clock signal, in which the conventional S-DRAM will malfunction if the time offset between DLL-CLK and VE-CLK is greater than one clock cycle time tcycle.
In order to avoid the malfunctioning of the read/latency time generator according to the prior art as described above, attempts have so far been made to minimize the signal delay times in order to reduce the delay time xcex94tDEC.
However, at very high clock frequencies, minimizing the signal delay times reaches its limits, and is not sufficient to prevent a malfunction.
The object of the present invention is thus to provide a latency time circuit for an S-DRAM which operates in a fault-free manner even with a very high-frequency clock signal, that is to say it emits a data enable signal with the desired latency time to the data path.
This object is achieved by a latency time circuit having the features specified in Patent claim 1.
The invention provides a latency time circuit for an S-DRAM, by means of which a high-frequency clock signal (CLK) is clocked, for producing a delayed internal data enable signal for synchronous data transfer through a data path of the S-DRAM, having:
a controllable latency time generator for delaying a decoded external data enable signal with an adjustable latency time,
with a comparison circuit being provided, which compares a cycle time (tcycle) of the high-frequency clock signal (CLK) with a predetermined signal delay time of the data path, and reduces the latency time of the latency time generator by the cycle time if the signal delay time of the data path is greater than the cycle time of the clock signal.
In one preferred embodiment of the latency time circuit according to the invention, a register is provided for storing a predetermined latency time.
Furthermore, a decoder is preferably provided, which decodes the stored latency time in order to produce an internal clock signal for the latency time generator.
In one preferred embodiment of the latency time circuit according to the invention, the comparison circuit emits a correction indication signal to the decoder if the signal delay time of the data path is greater than the cycle time (tcycle) of the clock signal (CLK).
In one particularly preferred embodiment of the latency time circuit according to the invention, the controllable latency time generator has a number of series-connected clocked time switching elements, each of which passes on a signal which is applied to a signal input, in each case delayed by the cycle time (tcycle), at a signal output.
The signal outputs of the time switching elements are preferably each connected to one signal input of a controllable multiplexer within the latency time generator.
The multiplexer preferably has a control input for the internal control signal which is emitted by the decoder.
The signal delay of all the series-connected time switching elements is preferably equal to a maximum programmable latency time minus two.
In one particularly preferred embodiment of the latency time circuit according to the invention, the decoder drives the multiplexer in the absence of the correction indication signal such that the signal delay which is produced by the time switching elements is equal to the programmed latency time reduced by two cycle times, and the decoder drives the multiplexer on receiving the correction indication signal such that the signal delay which is produced by the time switching elements is equal to the programmed latency time reduced by a further cycle time.
The latency time generator preferably has a synchronization circuit for synchronization of the decoded data enable signal to an internal clock signal.
In one particularly preferred embodiment of the latency time circuit according to the invention, the comparison circuit contains
a test signal generator for producing a test signal,
a data path delay time circuit, which delays the test signal that is produced by the signal delay time of the data path,
a clocked clock pulse generator for producing a clock pulse whose pulse duration is equal to the cycle time of the clock signal (CLK),
a transfer switching gate with an input for application of the delayed test signal,
a clock input for application of the clock pulse that is produced and having an output to which the test signal which is produced is passed on when the delay time of the delay time circuit is longer than the pulse duration of the clock pulse which is produced by the clock pulse generator, and
a latch circuit for temporary storage of the passed-on test signal.
The test signal is preferably a logic-high signal pulse with a long pulse duration.
The test signal which is passed on is preferably temporarily stored as a correction indication bit in the latch circuit.
The data path delay time circuit which is contained in the comparison circuit is preferably a circuit whose layout is identical to that of the data path.
The test signal generator preferably produces the test signal after receiving an enable signal from an internal sequence controller of the S-DRAM.
The data path which is contained in the S-DRAM preferably has a data FIFO register and a data output driver circuit (OCD).
The data path delay time circuit in a further embodiment comprises a number of series-connected delay elements, each of whose signal outputs can be passed on to a latch circuit through an associated transfer switching gate.
In one particularly preferred embodiment of the latency time circuit according to the invention, the comparison circuit is integrated in a DLL circuit of the S-DRAM.
Preferred embodiments of the latency time circuit according to the invention will be described in the following text with reference to the attached figures in order to explain features which are significant to the invention.